1. Field of the Invention
This invention relates to cellular telephone systems and, more particularly, to processes for detecting facilities which are providing degraded performance in cellular telephone systems.
2. History of the Prior Art
Presently available commercial mobile communication systems typically include a plurality of fixed base stations (cells) each of which transmits signals to and receives signals from mobile units within its communication area. Each base station is assigned a plurality of channels over which it can communicate with mobile units. A mobile unit within range of the base station communicates with the external world through the base station using these channels. Typically, the channels used by a base station are separated from one another in some manner sufficiently that signals on any channel do not interfere with signals on another channel used by that base station. To accomplish this, an operator typically allots to a base station a group of channels each of which is widely separated from the next. So long as a mobile unit is within the area in which the signal from a base station is strong enough and is communicating with only that base station, there is no interference with the communication.
In order to allow mobile units to transmit and receive telephone communications as the units travel over a wide geographic area, each cell is normally physically positioned so that its area of coverage is adjacent to and overlaps the areas of coverage of a number of other cells. When a mobile unit moves from an area covered by one base station to an area covered by another base station, communications with the mobile unit are transferred (handed off) from one base station to another in an area where the coverage from the adjoining cells overlaps. Because of this overlapping coverage, the channels allotted to the individual cells are carefully selected so that adjoining cells do not transmit or receive on the same channels. The channels used by adjoining base stations are also theoretically separated from the channels of each adjoining base station sufficiently that signals from any base station do not interfere with signals from another adjoining base station. This separation is typically accomplished by assigning a group of widely separated non-interfering channels to some central cell and then assigning other groups of widely separated non-interfering channels to the cells surrounding that central cell using a pattern which does not reuse the same channels for the cells surrounding the central cell. The pattern of channel assignments continues similarly with the other cells adjoining the first group of cells. The pattern is often called a channel reuse pattern.
There are a number of different types of mobile communications systems. Channels are defined in different manners in each of the different systems. In the most prevalent American Mobile Phone System (AMPS) system, channels are defined by frequency. A frequency band of 25 MHz providing approximately four hundred different adjoining FM frequency channels is allotted by the federal government to each cellular operator. In a typical AMPS system, each channel uses a fixed FM frequency band width of 30 KHz. for downlink transmission from a base station to a mobile unit and another fixed FM frequency band width of 30 KHz. for uplink transmission from a mobile unit to a cell. Typically, the frequencies assigned to the downlink transmissions for an entire cellular system immediately adjoin one another and are widely separated from the frequencies assigned to the uplink transmissions which also immediately adjoin one another.
Since channels are defined by frequency in an AMPS system, an operator typically allots to any single base station a set of channels with frequencies which are each separated from one another sufficiently to eliminate interference between those channels. In some AMPS systems, especially those with cells in urban areas carrying heavy traffic, each cell may be further divided into two or three sectors each of which may include channels having the above-described frequency allotment of channels. The antennas of each sector are typically arranged to provide 120 degree coverage. When cells are discussed herein, sectors are normally meant as well unless the context indicates otherwise.
Although the channels allotted to the individual cells are carefully selected so that adjoining cells do not transmit or receive on the same frequencies, it is very difficult to eliminate all interference in a system where channels are based on differences in frequencies. Antenna patterns, power levels, scattering, and wave diffraction differ from cell to cell. Buildings, various other structures, hills, mountains, foliage, and other physical objects cause signal strength to vary over the region covered by a cell. Consequently, the boundaries at which the signal strength of a channel falls below a level sufficient to support communications with a mobile unit vary widely within a cell and from cell to cell. For this reason, cells adjacent one another do not, in fact, typically form precise geometric boundaries. Because the boundaries of cells are imprecisely defined, signals will often interfere with one another even though they are generated by cells which are at distances theoretically sufficient to eliminate interference. This is especially true when a sectored cell pattern is used because the cells are much closer to one another than in a simple cell pattern.
Because of this interference, other types of mobile systems have been devised.
In one type of mobile system called Code Division Multiple Access (CDMA) digital signals are used to transmit data. All of the base stations of a CDMA system use the same "spread spectrum" frequency band of 1.25 megacycles to transmit the digital signals. The transmissions are combined with redundant channel coding information to allow error correction. The encoded signals are then multiplied by one of sixty-four Walsh codes which establish individual channels and increase the bandwidth to 1.25 megacycles. Because of the redundancy of the encoded signals, a receiver may decode a signal from the plethora of coded channels carrying data on the broad frequency band. Since the Walsh codes establish a number of individual channels and the pseudo-noise code assigned to each base station differs from those of other surrounding base stations, adjacent and remote cells may reuse the same frequency bands.
In another common type of mobile system called Time Division Multiple Access (TDMA), frequencies are assigned to the entire system in groups much like those of an AMPS system. However, within any frequency, each base station sends and receives in bursts during some number of different intervals or time slots. These time intervals within frequency bands then effectively constitute the individual channels. By assuring that the group of frequencies assigned to any individual base station differ from one another and from the frequencies assigned to base stations surrounding each individual base station, a channel reuse pattern is established which allows substantially greater use of the frequency spectrum because of the time division process.
In theory, these forms of cell arrangement and channel assignments allows channel reuse patterns to be repeated at distances separated sufficiently to negate interference between mobile units on the same and adjacent channels. They also allow signals used by the different cells in a particular system to be strong enough to provide complete coverage of the system area. In fact, interference and insufficient coverage do occur.
One reason that interference and insufficient coverage occur in the various mobile systems is that the various components of the system deteriorate or fail in some way so that cells operate at less than optimum. This may reduce the signal strength provided by certain cells thereby changing the patterns from those originally planned for the system. Such pattern changes reduce coverage in some areas and may cause undesired overlap of signals from remote cells. Component failures may be rapid or slow. Antennas are often physically damaged. This can cause a lower level response or actually shift the antenna pattern. Connections loosen or otherwise become less than optimum so that lower levels of current are transferred. Any number of other causes may affect the level at which a cell transmits, the direction of that transmission, or other characteristics of the transmission. When a cell is transmitting or receiving at less than optimum, the tolerances expected in the system may no longer exist thereby causing spurious signals to reach the same level as desired signals.
Isolating a cell which is experiencing reduced functionality is very difficult because of the interaction of the signals over an entire cellular system. Heretofore, it has been necessary to physically visit and inspect each cell which might provide signals to an area where difficulty is being experienced in order to determine whether damage has occurred to the components of the individual cells. Such visits require the physical inspection of the cell components and the measurement of the various characteristics of the cell. Usually the measurement of the various characteristics of the cell must be performed by a skilled technician. Testing a system to determine what is causing interference or coverage reduction is extremely labor intensive and time consuming. Testing a system is consequently expensive.
It is desirable to provide a process by which the deterioration and failure of components of cells in a mobile telephone system may be detected rapidly, inexpensively, and without the investment of substantial amounts of expensive labor.